1. Field of the Invention
The present invention relates to platens for hydroforming finished plastic sheet material and, more particularly, to a diaphragm and circumscribing seal of the platen.
2. Description of Related Art
The process of hydroforming has been practiced for many years. It includes the use of a flexible diaphragm disposed in a cavity of a platen for forcing sheet material to conform with the configuration of a mold. Hydraulic fluid, acting upon the diaphragm, causes the diaphragm to force the sheet material disposed intermediate the diaphragm and the mold to conform with the mold. Because substantial pressures, sometimes exceeding 5,000 pounds per square inch, may be present, leakage of hydraulic fluid about the edges of the diaphragm and onto the mold or sheet material to be formed may occur. In hydroforming operations involving a subsequent step in fabricating a product, cleaning is the norm and leakage of hydraulic fluid onto the product molded is of minor concern. When the product molded is to be a finished product, contamination of the product by leaking hydraulic fluid creates a significant problem of requiring an additional cleaning step and possibly compromising the product.
To prevent leakage of hydraulic fluid adjacent the perimeter of the diaphragm and to maintain the pressurized hydraulic fluid within the platen cavity behind the diaphragm, it has been standard practice to place a seal in a channel surrounding the diaphragm. The seal is formed of four rails having angled ends to mate in the manner of a conventional picture frame. The seal is machined with high tolerance to fit the corresponding slots, and grooves in a channel about the diaphragm. The machining must be very accurate in order to develop a leak proof junction between the seal, the channel and the perimeter side of the diaphragm. Moreover, the pressures involved are sufficient to cause lateral migration of the material of the diaphragm unless such migration is halted by the seal. If migration does occur, it will result in ultimate destruction of the diaphragm. From time to time, the seal will wear or become damaged and must be replaced. Such replacement requires meticulous hogging out of the damaged seal, machining a new seal to conform with the channel for the seal and the adjacent diaphragm and installing the replacement seal. The expenses and lost time attendant seal replacement are not insignificant.